Muscle tissue is contractile tissue. It is
responsible for movement in the organism. There are three types of
muscle tissue found in the animal kingdom (striated, cardiac, and
smooth). Muscle tissue is grouped in two ways: structure of the cells
and whether it is under the control of our conscious or unconscious
mind.

Skeletal Muscle. This muscle is attached to
the organisms skeleton. Its major responsibility is to move the
organism. These muscles are paired up in such a way as there
movements oppose each other. This is called working in
antagonistic pairs. Skeletal muscles contract actively, while
the same muscle will relax passively. If a limb is moved in one
direction actively, it cannot get back to its original place by just
relaxing. Another muscle must contract to pull it back to its
original place. The elbow joint is moved by the alternate contraction
and relaxation of the biceps and triceps muscles. These muscles are
under the control of the cerebrum. They
are called voluntarymuscles.
Muscle coordination is under the control of the cerebellum.
The cerebrum allows us to control the movement of a part of the body
but it's the cerebellum that allows us to walk, pick up things, and
control these movements in a coordinated manor.

Skeletal Muscle Structure.The cells of
skeletal muscles are long fiber-like structures. They contain many
nuclei and are subdivided into smaller structures called
myofibrils. Myofibrils are composed of 2 kinds of
myofilaments. The thin filaments are made of 2 strands of the
protein actin and one strand of a
regulatory protein coiled together. The thick filaments are staggered
arrays of myosin molecules.

Units of organization of skeletal muscle.
The filaments are organized into structures called sarcomeres.
Sarcomeres are constructed in the following manner:

Z lines are at the borders of the sarcomere. They
align in adjacent myofibrils.

I bands are areas near the edge of the sarcomere
containing only thin filaments.

A bands are regions where thick and thin filaments
overlap and correspond to the length of the thick
filaments.

H zones are areas in the center of the A bands
containing only thick filaments.

The Mechanism of Muscular contraction.
Muscle contraction reduces the length of each sarcomere. The sliding
filament theory is explained below.

The thin filaments ratchet across thick filaments
to pull the Z lines together and shorten the sarcomere. The
myofilaments themselves do not contract.

Myosin molecules on thick filaments attach to the
actin on the thin filaments to form a crossbridge. The cross bridges
then bend inward pulling the thin filament toward the center of the
sarcomere. These cross bridges are broken and reformed further down
.

Energy for cross bridge formation comes from the
hydrolysis of ATP by the head region of the myosin.

Skeletal muscle contracts when stimulated by motor
neurons.

Graded Contraction of Whole
Muscles. are due to
summation of multiple motor unit activity and wave
summation.

Motor neurons usually deliver their stimuli
rapidly, resulting in a smooth contraction.

A motor unit consist of a single motor neuron and
all the muscle fibers it controls.

As more motor neurons are recruited by the brain,
tension in the muscle progressively increases.

Duration of the muscle contraction depends on how
long the concentration of calcium remains elevated.

Slow fibers have longer lasting twitches
because they have less sarcoplasmic reticulum. The calcium remains in
the cytoplasm longer. They have many mitochondria, rich blood supply,
and myoglobin.

Fast fibers have short duration twitches
and found in fast muscles for rapid, powerful
contractions.

Cardiac Muscle. in vertebrates is only
found in the heart. It is striated. Muscles cells are branched, and
the junctions between the cells contain intercalated discs that
electrically connects all heart muscle cells, allowing coordinated
action. Cells can also generate their own action potential.

Smooth Muscle. lack striations and contain
less myosin; the myosin is not associated with specific actin
strands. Contractions are slow but there is a greater range of
control. Calcium must enter the cell through the plasma membrane
during an action potential. Found in the walls of blood vessels and
the digestive organs.